Batch type industrial heat treat furnaces may be generally defined as either i) positive pressure furnaces which operate at about standard atmospheric pressure and are generally box shaped or ii) vacuum furnaces (which includes plasma or ion furnaces) which heat the work under a vacuum and are generally cylindrical pressure vessels employing a double wall vacuum tight casing defining a cooling water jacket therebetween. In both furnace types, a sealable door is provided for access to the furnace chamber to load batches of work onto a hearth. The work is heated, and a furnace atmosphere treating gas is introduced (during or after heating) and the work is cooled in a specified manner or cycle to effect a desired heat treatment. Certain heat treatment processes dictate use of a vacuum during some period of the cycle. As used herein, "vacuum furnace" means a furnace that pulls a vacuum in the furnace chamber during any portion of a heat treat cycle. For example, if a vacuum is used only to purge the furnace chamber prior to performing a heating and cooling heat treat process at positive pressure, the furnace is a vacuum furnace.
Positive pressure furnaces are less costly than vacuum furnaces primarily because only one furnace casing, which does not have to be welded vacuum tight is provided. Typically, the box furnace is lined with insulation on its inside so that the insulation is at furnace temperature while the casing exposed to ambient atmosphere, is at a far lesser temperature, but typically higher than ambient, hence its designation as a "hot wall" furnace. Providing the casing on the outside allows door sealing to be readily achieved between door flange and casing. However, the hearth sits on supports anchored to the casing and undergoes differential thermal expansion requiring an expansion joint construction. The assignee has sold a box type, positive pressure furnace in which the furnace insulation was applied to the outside of the casing. This allows for an integral hearth construction but resulted in door sealing concerns at the operating temperatures of the furnace which are best addressed by the provisions of a water cooled seal such as disclosed in the '857 patent for the radiant tube illustrated therein.
There are furnace applications where a portion of the heat process, such as tempering or work cleaning, is economically justified on a throughput basis, to be performed in a separate low cost furnace. These tempering or draw furnaces, which are low cost intensely cost competitive furnaces, are typically positive pressure furnaces using convective heat transfer to rapidly heat the work by circulating the furnace wind mass vis-s-vis movable or stationary baffles or damper arrangements. The assignee that such arrangements were unwieldy and introduced a cylindrical furnace under its UniDraw.RTM. brand name disclosed in the '091 patent to produce a wind mass pattern which heated the work at significantly better temperature uniformities than previously achieved.
Subsequently, the assignee determined that the single, cylindrical casing of the Uni-Draw furnace can be welded vacuum tight and the furnace functions as a vacuum furnace. As noted in the '976 patent, there are several heat treat processes which do not require high vacuum (low pressure) levels typically pulled by conventional, double walled, water jacketed vacuum furnaces. At these "soft" vacuum levels, the Uni-Draw furnace, modified to produce different wind mass patterns, special provisions for quenching and a single, vacuum tight casing as explained in the '782 and '985 patents (and marketed by assignee under its VacuDraw.RTM. brand name) has successfully functioned as a "hot wall" vacuum furnace.
The construction of the VacuDraw furnace is conventional in that a furnace casing is provided and batts or mats of furnace insulation are applied to the interior of the casing which is vacuum tight. A metal skin (oven panel) may optionally be applied to the exposed inner surface of the insulation or alternatively, a silicate rigidizer, i.e., Kaowool rigidizer, may be sprayed over the exposed surface of the fibre ceramic insulation. The thickness of the insulation determines the temperature of the furnace casing. Thus, the furnace casing does not significantly undergo thermal expansion and contraction and conventional elastomer sealing arrangements can be used for vacuum sealing the furnace door furnace "components" entering into the furnace chamber from the outside of the furnace without the need for water jacket cooling.
In the low temperature ranges of the temper or draw furnaces, the assignee has discovered that a VacuDraw furnace has a particularly useful advantage by initially pumping out the atmosphere in the furnace before introducing the treating gas to avoid purging the furnace with an inert atmosphere. After the vacuum is drawn the treatment gas is backfilled to place the furnace chamber at positive pressure so that convective heating of the work can occur. The cycle time is significantly decreased and the costs are reduced by eliminating the expense of an inert purge gas with an inexpensive furnace. In certain applications, the furnace is pumped down after heating with the work hot and the furnace atmosphere changed. However, it is believed that not all the elements of the furnace atmosphere are drawn out by the soft vacuum. Certain gaseous compounds can migrate into the furnace insulation before or during heating and become trapped. On cool down or heat up, the gases form undesirable compounds or contaminates which could effect the process, i.e., water vapor or oils from dirty parts.